a. Field Of The Invention
The present invention relates to various novel apparatus and processes for mining subterranean deposits of granular ore such as phosphate rock or any other similar material.
B. Description of The Prior Art
Subterranean deposits have been found that cannot be mined by conventional mining apparatus or methods or by dredging. For example, phosphate ore bodies have been found at depths ranging from 150 to 300 feet or more below the ground surface. This is far beyond the reach of dredges and conventional mining methods are inapplicable because the water table is usually encountered a short distance below ground level, e.g., 5 to 10 feet in some cases.
Various methods have been used heretofore in attempts to mine granular ore deposits existing so far below ground level. One method that has been attempted involved the use of compressed air for forcing the slurry to the ground level. The problem with this system has been the lack of suitable controls on the slurry level in relation to the mining head resulting in many cases in blowbacks up through the tool string and in rapid depressurization in the ore cavity. These irregularities also prevent proper density control. Not only did this waste time and work, it could result in the loss of expensive equipment because of overburden subsidence due to a sudden increase in differential pressure across the seal between the water string and the overburden. The lack of blowback prevention also complicates the addition or removal of sections from the tool string after the ore cavity has been pressurized with air and usually would require prior depressurization resulting in loss of compressed air and additional time entailed in depressurization and subsequently repressurizing.
Pfefferle U.S. Pat. No. 3,439,953 discusses the problems of sudden pressure loss in air pressurized ore cavities and teaches mining while the cavity is full of water until a suitable sized sump is created to facilitate control of the slurry level and prevent it from dropping below the eductor tube through which slurry is sent to the surface. There is no mention in this patent of a valve for the purpose of closing the slurry return passage. Furthermore, the tool string used in the Pfefferle apparatus employs four separate pipes or tubes extending from the ore cavity to ground level and this considerably complicates the assembly and disassembly of the sections of the tool string. In the absence of blowback controls, the Pfefferle apparatus does not permit addition or removal of sections while the ore cavity remains pressurized or the conservation of the pressurized air upon cessation of mining in the cavity for use in another cavity.
Pumps based on the venturi principle have been tried in the mining head. These attempts are exemplified in Fly U.S. Pat. Nos. 3,155,177 and 3,316,985 and in Wenneborg 3,747,696. No provision is made in any of these patents, however, for the prevention of blowbacks by means of a downhole valve which controls the slurry return passage to close it in the event the slurry level in the ore cavity reaches a predetermined low level. The apparatus of the Fly patents utilizes a downhole slurry level detector which sends a signal to ground level for the purpose of actuating a control mechanism to regulate the throttle valve of a diesel engine which drives a pump delivering water down the hole. An arrangement of this type functions to deliver more water down the hole when the slurry level in the ore cavity drops and less when it rises. The Fly apparatus is not readily adapted to be used in systems utilizing pressurized air in the ore cavity and is unable to gain the advantages of such systems including the cavity wall support and the provision of pumping and air lift assistance characteristics of such systems. Also, the Fly apparatus and process requires the transmission of an electrical signal from the slurry level in the ore cavity to the ground surface which necessitates the provision of special insulation against grounding and imparts the risk of faulty signals or disrupted signals due to accidental grounding or shorting.
The Wenneborg device incorporates a drilling bit and jet mining nozzles. It also is not readily adaptable for use in pressurized air systems and lacks any blowback prevention arrangement. In addition the Wenneborg device utilizes a more complicated tool string which includes an extra water line and thus renders more difficult the addition of sections as the tool string is lowered. Furthermore, the Wenneborg device does not appear to have any means for measuring or controlling the slurry level in the ore cavity.
Donaldson et al U.S. Pat. No. 3,030,086 uses an air lift for raising the slurry to the surface. The tool string used in this apparatus is very complex and difficult to assemble when adding or removing sections. It also fails to provide any blowback prevention device and is not well adapted to use in air-pressurized ore cavities.
Other patents relating to apparatus for the mining of granular ores and similar apparatus but appearing to be less pertinent include U.S. Pat. Nos. 811,275; 908,113; 931,057; 1,960,932; 2,019,968; 2,127,265; 2,210,582; 2,376,974; 2,412,765; 2,518,591; 2,599,980; 2,678,203; 2,720,381; 2,745,647; 2,772,868; 3,153,290; 3,311,414 and 3,797,590.